This invention relates to metallurgical bonding methods and more particularly to welding or brazing methods for welding or brazing a sleeve within a tube.
In tube-type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Occasionally, one of the tubes can become defective such that a leak occurs therein which allows the fluids to mingle. When this occurs, it is sometimes necessary to either plug the tube so that the fluid does not flow through the tube or repair the tube, thereby preventing leakage from the tube.
In nuclear reactor power plants, the tube-type heat exchangers are commonly referred to as steam generators. When a defect occurs in a tube of a nuclear steam generator that allows the coolant in the tube to mingle with the coolant outside of the tube, a more significant problem arises. Not only does this situation create an ineffective heat exchanger, but it also creates a radio-active contamination problem. Since the fluid flowing in the tubes of the nuclear steam generator is generally radioactive, it is important that it not be allowed to leak from the tubes and contaminate the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube must either be plugged or repaired so that the coolant does not leak from the tube. This prevents contamination of the fluid surrounding the tubes.
There are several methods known in the art for repairing heat exchange tubes; however, many of these methods are not applicable to repair of heat exchange tubes wherein the tube is not readily accessible. For example, in a nuclear steam generator the physical inaccessibility of defective heat exchange tubes and the radioactive nature of the environment surrounding the heat exchange tubes presents unique problems to repairing heat exchange tubes that do not normally exist in other heat exchangers. For these reasons, special methods have been developed for repairing heat exchange tubes in nuclear steam generators. Typically, the method used to repair a heat exchange tube in a nuclear steam generator is one in which a metal sleeve having an outside diameter slightly smaller than the inside diameter of the defective tube is inserted into the defective tube and attached to the defective tube to bridge the defective area of the tube. This type of repair method is generally referred to as "sleeving". Previous sleeving development work has been concerned with obtaining a relatively leakproof joint between the sleeve and the tube by brazing, arc welding, explosive welding, or other joining means. Due to the need for cleaniness, close fittings, heat application, and atmospheric control, these metallurgical bonding techniques have problems which are not easily solvable in areas such as a nuclear steam generator where human access is limited.
In the braze sleeving methods such as the one described in U.S. patent application Ser. No. 185,654, filed Sept. 9, 1980 in the name of R. D. Burack and entitled "Gold Braze Sleeving Method" which is assigned to the Westinghouse Electric Corporation, it is necessary to heat the braze metal in order to form the braze bond between the sleeve and the tube. One way to heat the braze material is by inserting a heating apparatus in the sleeve so as to internally heat the sleeve and the braze material. However, due to the inaccessibility of the work area, the power requirements for the heating apparatus, and the need to carefully control the brazing times and temperatures, a specially designed internal brazing wand is recommended for use in such a process.
In welding methods for internally welding sleeves to tubes in heat exchangers, special problems arise that must be solved in order to establish an effective weld joint between the sleeve and the tube. For example, commonly the sleeve is internally expanded into contact with the tube for establishing a contact surface between the sleeve and the tube for either brazing or welding. However, despite the internal expansion of the sleeve against the tube, it is not always possible to achieve a consistently uniform contact between the sleeve and the tube. When subjected to an internal welding arc, the non-uniform contact between the sleeve surface and the tube surface can lead to a non-uniform thermal contact resistance. Near the location of intimate contact between the sleeve and the tube, the heat from the welding arc is conducted more readily away through both sleeve and the tube than it does about the portion where there is no contact therebetween. At the non-contact segment of the sleeve-tube interface, the arc's heat must be dissipated by the members' thin walls. Because of the limited "two-dimensional" heat conduction, the heat tends to accumulate about the fusion zone and slows down the solidification rate of the molten pool. As the surface tension of the pool is lowered inversely to its temperature, it might be overcome by the arc force and rupture the molten pool. This allows the arc to pierce through the sleeve and directly impinge upon the internal surface of the tube. Even if the weld pool is not ruptured by this mechanism, limited conductivity from the weld pool to the external tube may result in erratic fusion between the sleeve and the tube.
Another problem that may develop in internally welding a sleeve to a tube is that as the sleeve begins to be welded to the tube, the sleeve may tend to be pulled toward the side of the tube where the welding is initiated causing a gap to develop diametrically opposite to that point between the sleeve and the tube. This may result in a non-uniform contact between the sleeve and the tube with resultant non-uniform thermal contact resistance.
Moreover, distortion between the tube and the sleeve may be caused by stress relief of the expanded sleeve. Because the expanded sleeve has residual stresses locked into its structure, the sleeve can distort as it is heated thereby relieving the stress. As a result, misalignment between the sleeve and tube and non-uniform contact between the sleeve and the tube may result.
An optimized sleeve-to-tube joining method would assure consistently uniform temperature distribution and arc force between the sleeve and the tube at their interface, which would alleviate the non-uniform contact problem. Therefore, what is needed is an internal sleeve-to-tube joining method wherein a substantially uniform contact between the sleeve and tube is maintained thereby establishing a substantially uniform thermal contact resistance between the sleeve and the tube to produce a quality joint therebetween.